The HU-3 is the optional heating unit for the SL-QDM Series, SL-QDP Series, SL-QN Series, AX-TN Series, and AX-TF Series Photoelectric Detectors. It comes as a set (one for transmitter beam and one for receiver beam - 2 sets required for SL Series beams) and should be used in temperatures below freezing or where frost is an issue.